This advanced concept hybrid helicopter is described in document FR 2 916 418 and it combines, at reasonable cost, the effectiveness in vertical flight of a conventional helicopter with the high travel speed performance made possible by using propulsive propellers and the installation of modern turbine engines.
Thus, a hybrid helicopter is neither a helicopter, nor an autogyro, nor a gyrodyne. Similarly, a hybrid helicopter is neither a compound nor a convertible aircraft.
A hybrid helicopter comprises a fuselage, and a rotary wing having a main rotor for driving blades in rotation by means of at least one turbine engine.
The hybrid helicopter is also provided with a wing made up of two half-wings, with two propulsive propellers being placed on either side of the fuselage, on the half-wings.
In addition, the hybrid helicopter is fitted with an integrated drive system that includes not only the turbine engine(s), the main rotor, and the two propellers, but also a mechanical interconnection system between these elements.
With this configuration, the speeds of rotation at the outlet(s) of the turbine engine(s), of the propellers, of the main rotor, and of the mechanical interconnection system are mutually proportional, with the proportionality ratio being constant regardless of the flight configuration of the hybrid helicopter under normal operating conditions of the integrated drive system.
Consequently, and advantageously, the main rotor is always driven in rotation by the turbine engine(s) and it always develops lift, whatever the configuration of the hybrid helicopter, both in forward flight and when hovering.
More precisely, the main rotor is designed to provide all of the lift for the hybrid helicopter during stages of takeoff, of landing, and of vertical flight, and to provide part of its lift during cruising flight, with the wing then contributing lift for supporting said hybrid helicopter.
Thus, the main rotor provides the major fraction of the lift of the hybrid helicopter in cruising flight and possibly makes a small contribution to propulsive or traction forces while always being in a minimum drag configuration.
Like a helicopter, the pilot then has first and second control means for controlling respectively the collective pitch and the cyclic pitch of the blades of the main rotor.
Furthermore, by collectively modifying the pitch of the blades of the propellers of the hybrid helicopter by the same amount, it is also possible to control the thrust generated by the propellers.
Thus, the pilot has at least one thrust control means suitable for collectively modifying the pitch of the blades of the propellers by the same amount.
In contrast, the anti-torque and steering functions are provided by using differential thrust exerted by the propellers, e.g. by the pilot operating a rudder bar.
Consequently, the thrust control means enables the mean pitch of the blades of the first and second propellers to be defined, said mean pitch corresponding to half the sum of the first and second pitches of the blades of the first and second propellers.
In contrast, the rudder bar serves to cause the pitches of the blades of the first and second propellers to depart from the mean pitch in differential manner, with the pitch of the blades of one propeller being increased by a certain amount while the pitch of the blades of the other propeller is decreased by the same amount.
It will be understood that it can then be difficult to pilot the hybrid helicopter insofar as there are multiple controls to be operated. Furthermore, it is appropriate to avoid giving orders to the first and second propellers that might degrade the behavior of the helicopter, e.g. by the propellers generating a resultant thrust on the hybrid helicopter in a rearward direction.
Document U.S. Pat. No. 5,050,081 describes an indicator presenting the thrust generated by an aeroengine as a percentage of the maximum thrust available from said engine.
Similarly, document EP 1 471 334 presents an indicator graduated in percentage of the maximum thrust of an engine, which indicator displays the actual thrust and also the thrust requested of the engine.
Although effective, the teaching of those documents is not transposable to a hybrid helicopter. On a hybrid helicopter, thrust is generated by the first and second propellers and also by the main rotor, and not directly by an engine. The technical field of the invention is thus remote from that of the above-mentioned document.
Patent FR 2 756 256 presents an indicator that displays the power margin available from a helicopter turbine engine on an indicator that is graduated in degrees for the collective pitch of the blades of the main rotor.
Nevertheless, that teaching does not make it possible to provide a piloting assistance device for a hybrid helicopter that possesses not only a main rotor but also first and second propellers that are linked to the main rotor by an interconnection system. The special nature of a hybrid helicopter means that an order given to the main rotor has consequences on the first and second propellers, and vice versa, which means that the known state of the art cannot be applied thereto.
Document U.S. Pat. No. 4,514,142 presents an aircraft provided with an engine installation, a main rotor, and a rear thruster arranged at the rear of a tail boom.
According to that document, the total torque available from the engine installation is measured in real time and the torque opposed by the main rotor is subtracted therefrom. More particularly, a first signal is generated in the form of a voltage that is delivered to means for detecting the main rotor torque so that said detector means generate a second signal.
The first and second signals are then optionally delivered to a display that displays variation in the remaining torque available for the rear thruster as a function of time. The difference is then presented between the available engine torque minus the rotor torque.
Although effective, it would appear difficult to make use of such a device in practice to facilitate piloting the aircraft.